1. Field of the Invention
The present invention relates to jigs for use in CVD for film formation, low pressure CVD methods using the same, and methods of manufacturing the CVD jigs, and particularly to a jig made of quartz for use in CVD, a low pressure CVD method using it, and a method of manufacturing the low pressure CVD jig.
2. Description of the Background Art
FIG. 12 is a perspective view showing the structure of a quartz boat. The quartz boat 1 is formed of three or more (four in this diagram) wafer supporting members 2 for supporting wafers 4 and quartz plates 3 for standing the wafer supporting members 2. The wafer 4 shown by the dotted line is placed in grooves of the wafer supporting members 2. Since contamination of the wafers 4 is undesirable, the wafer supporting members 2 are formed of quartz as well. For example, the contaminants include metal such as sodium, iron, etc. FIG. 13 is a diagram showing, in an enlarged manner, part of the wafer supporting member 2 in FIG. 12. The wafers are placed in the grooves 5.
Next, a method of manufacturing the wafer supporting members 2 will be explained. Prepared first is a stick-like base material of quartz. The base material of quartz prepared has no grooves 5 yet. The grooves 5 are formed by using a diamond cutter, for example.
FIG. 14 shows a section of the wafer supporting member 2 after the grooves are formed with a diamond cutter. In this stage, there are micro cracks 6 and abrasions 7 of about 10 .mu.m in the surface of the wafer supporting member 2.
Conventionally, as described in Japanese patent Laying-Open Gazette No.52-45260, the surface of the wafer supporting member 2 is directly burned with a flame of an oxyhydrogen burner 8 shown in FIG. 15 so that the surface of the wafer supporting member 2 melts and the micro cracks 6 and the abrasions 7 disappear. Since the temperature of the flame of the oxyhydrogen burner 8 reaches as high as 2000.degree. C., the trace of the flame is rapidly cooled and therefore rehardened and an affected layer 9 is formed as shown in FIG. 16. The affected layer 9 is formed of quartz having a different crystal structure than the base material, which can be observed by the following methods. For example, it can be observed by using a polarizing microscope, or a scanning electron microscope utilizing the difference in etching and cleavage properties, or the Fourier transform infrared spectroscopy (FT-IR).
Next, a film formation process by the low pressure CVD using the CVD jig will be explained referring to FIG. 17.
First, in Step ST1, wafers are placed on the quartz boat 1 shown in FIG. 12. In Step ST2, the quartz boat 1 is placed in a certain position in a CVD device.
In Step ST3, gases are introduced into the CVD device to expose the wafers, together with the quartz boat 1, to a desired atmosphere to form a film on the surfaces of the wafers. A film is also formed on the surface of the quartz boat in the film formation process. Since the conditions for the thin film formation require temperatures as high as 500.degree.-800.degree. C., for example, and it is necessary to prevent contamination of the wafers, quartz glass or ceramics are used as the material of the boat.
In Step ST4, the wafers having the films are taken out together with the quartz boat 1. At this time, it is determined whether it is necessary to remove the film formed on the surface of the quartz boat 1 (Step STS). This determination in Step ST5 is made not only by directly observing the surface condition of the quartz boat 1 but also by considering indirect conditions, such as the integrated number and integrated time of the film formation processes.
If it is determined in Step ST5 that a film removing treatment is not required, the process returns to Step 1 to continue the film formation process. When it is determined that the film must be removed, however, the quartz boat 1 is dipped into a cleaning chemical such as hydrofluoric acid, hydrochloric acid, nitric acid, or the like, to remove the film attached to the surface of the quartz boat.
The conventional CVD jig, the low pressure CVD method using it, and the method of manufacturing the CVD jig are structured as explained above. Since the jig is burned with an oxyhydrogen burner to prevent frosting and cracks, the OH concentration in the quartz glass is increased. This is due to the fact that in the conventional melting process with an oxyhydrogen burner 8, hydrogen and quartz in the wafer supporting member 2 react to produce SiO.sub.a (OH).sub.b. Where "a" is larger than 0 and smaller than 2, or "b" is larger than 0 and smaller than 4, and the relation 2a+b=4 is satisfied. When hydrofluoric acid (HF) is used as a cleaning chemical, for example, the reaction shown in the formula 2 proceeds faster than that shown in the formula 1. Hence, it becomes more susceptible to corrosion than the case of a lower OH concentration. Then the surface conditions of the quartz are deteriorated to cause dust, which reduces the yield when semiconductor devices are mass-produced by using this kind of jigs. ##EQU1##
Especially, in recent years, for the purpose of removing the films attached on the surfaces, the CVD jigs are often treated in a vapor phase by using a cleaning chemical of chlorine trifluoride (ClF.sub.3), nitrogen trifluoride (NF.sub.3), for example. This brings about more serious deterioration of the surface conditions of the CVD jigs made of quartz.
FIG. 18 is a diagram showing occurrence of dust, wherein the film 10 formed on the surface of the wafer supporting member 2 comes off in the region 11 to be a piece of dust 12. FIGS. 19A and 19B provide pictures of the dust 12 taken with a scanning electron microscope. These pictures show that the dust have a size of about several micrometers. Since the thickness of the film is less than 1 .mu.m, it is seen that the quartz has come off together with the film. FIG. 20 is a side view of a section showing the relation between the wafer 4 and the wafer supporting members 2. The dust comes off from the wafer supporting members 2 and scatters along the tracks shown by the arrows 13. However, note that a plurality of pieces of dust do not always occur at the same time. Only one may occur, or most probably, no dust will occur at all. However, the quartz comes off in repeated use and then the quartz jig can not be used many times.
The difference between the coefficient of linear expansion of the attached film 10 and that of the quartz glass forming the wafer supporting member 2 also contributes to the dust. Table 1 shows the coefficients of linear expansion of materials under the same conditions as to the number of applications and the number of cleanings. As the difference in the coefficients of linear expansion increases, and as the difference in temperature for film formation increases, the film 10 comes off more frequently.
While Table 1 shows the values with bulk materials, films formed by CVD generally provide larger values.
TABLE 1 ______________________________________ Material SiO.sub.2 Si C SiN SiC AlN Al.sub.2 O.sub.3 ______________________________________ Coef of linear 0.5 2.2-2.4 3.2 3.4 4.3 4.5 8.0 expansion .times. 10.sup.-6 / deg. ______________________________________ Note: SiO.sub.2...quartz, Si...silicon, C...carbon, SiN...silicon nitride, SiC...silicon carbide, AlN...aluminum nitride, Al.sub.2 O.sub.3...alumina.
Note: SiO.sub.2 . . . quartz, Si . . . silicon, C . . . carbon, SiN . . . silicon nitride, SiC . . . silicon carbide, AlN . . . aluminum nitride, Al.sub.2 O.sub.3 . . . alumina.